Most of today's bioanalytical methods, such as immunoassays or hybridisation assays, require the use of labels, such as fluorescent labels, for the detection of the analyte. The use of such labels is, however, undesirable for a number of reasons: The necessity of washing the sample prior to the measurement is time-consuming and may also prevent the observation of transient interactions, low affinity binding events and binding kinetics. Moreover, the introduction of labels is costly and may alter the biological system under study in an undesirable and sometimes unpredictable manner. Finally, insufficient knowledge of the labeling efficiency may result in false results in biomedical analysis.
One technique allowing the label-free detection of an analyte involves surface plasmon resonance (SPR). The surface plasmon is a propagating wave of collectively oscillating free electrons at an interface between a thin metallic film and a dielectric material. As the propagation of the wave is influenced by the dielectric constant at the interface, adsorption processes at the interface can be sensitively detected. SPR sensors are commercially available, e.g. under the trade name BIAcore from Pharmacia Biosensor, Piscataway, N.J., USA.
A major limitation of these SPR sensors is the fact that the surface plasmon in e.g. gold layers propagates over comparably large distances, prohibiting their use for the detection of a plethora of different analytes that is needed in parallel, miniaturized biological assays such as protein arrays or DNA arrays.
Proposals have been made to use gold nanoparticles to exploit the phenomenon of localized or colloidal SPR. WO03/050291 describes a sensor obtained by depositing gold or silver nanoparticles on optically transparent substrates such as glass. US 2003/0174384 A1 describes the use of so-called gold nanoshells on top of a gold surface.
EP 0 965 835 A2 and Sensors and Actuators B 2000, 63, pp. 24-30 describe the formation of a monolayer of polystyrene particles on which a gold coating was subsequently formed by thermal evaporation, thereby forming cap-shaped gold nanoparticles. These sensor chips are reported to show a good sensitivity.